lib/a_star.nit

   1 # This file is part of NIT (http://www.nitlanguage.org).
   2 #
   3 # Copyright 2011-2013 Alexis Laferrière <alexis.laf@xymus.net>
   4 #
   5 # Licensed under the Apache License, Version 2.0 (the "License");
   6 # you may not use this file except in compliance with the License.
   7 # You may obtain a copy of the License at
   8 #
   9 #     http://www.apache.org/licenses/LICENSE-2.0
  10 #
  11 # Unless required by applicable law or agreed to in writing, software
  12 # distributed under the License is distributed on an "AS IS" BASIS,
  13 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  14 # See the License for the specific language governing permissions and
  15 # limitations under the License.
  16
  17 # Services related to pathfinding of graphs using A*
  18 # A single graph may have different properties according to the `PathContext` used
  19 #
  20 #
  21 # Usage:
  22 #
  23 # ~~~
  24 # # Weighted graph (letters are nodes, digits are weights):
  25 # #
  26 # #     a -2- b
  27 # #    /     /
  28 # #   3     1
  29 # #  /     /
  30 # # c -3- d -8- e
  31 # #
  32 # var graph = new Graph[Node,WeightedLink]
  33 #
  34 # var na = new Node(graph)
  35 # var nb = new Node(graph)
  36 # var nc = new Node(graph)
  37 # var nd = new Node(graph)
  38 # var ne = new Node(graph)
  39 #
  40 # var lab = new WeightedLink(graph, na, nb, 2)
  41 # var lac = new WeightedLink(graph, na, nc, 3)
  42 # var lbd = new WeightedLink(graph, nb, nd, 1)
  43 # var lcd = new WeightedLink(graph, nc, nd, 3)
  44 # var lde = new WeightedLink(graph, nd, ne, 8)
  45 #
  46 # var context = new WeightedPathContext(graph)
  47 #
  48 # var path = na.path_to(ne, 100, context)
  49 # assert path != null else print "No possible path"
  50 #
  51 # assert path.step == nb
  52 # assert path.step == nd
  53 # assert path.step == ne
  54 # assert path.at_end_of_path
  55 # ~~~
  56 module a_star
  57
  58 # General graph node
  59 class Node
  60         type N: Node
  61
  62         # parent graph
  63         var graph: Graph[N, Link]
  64
  65         init(graph: Graph[N, Link])
  66         do
  67                 self.graph = graph
  68                 graph.add_node(self)
  69         end
  70
  71         # adjacent nodes
  72         var links: Set[Link] = new HashSet[Link]
  73
  74         # used to check if node has been searched in one pathfinding
  75         private var last_pathfinding_evocation: Int = 0
  76
  77         # cost up to in current evocation
  78         # lifetime limited to evocation of `path_to`
  79         private var best_cost_up_to: Int = 0
  80
  81         # source node
  82         # lifetime limited to evocation of `path_to`
  83         private var best_source: nullable N = null
  84
  85         # is in frontier or buckets
  86         # lifetime limited to evocation of `path_to`
  87         private var open: Bool = false
  88
  89         # Main functionnality, returns path from `self` to `dest`
  90         fun path_to(dest: N, max_cost: Int, context: PathContext): nullable Path[N]
  91         do
  92                 return path_to_alts(dest, max_cost, context, null)
  93         end
  94
  95         # Find a path to a possible `destination` or a node accepted by `alt_targets`
  96         fun path_to_alts(destination: nullable N, max_cost: Int, context: PathContext,
  97                 alt_targets: nullable TargetCondition[N]): nullable Path[N]
  98         do
  99                 var cost = 0
 100
 101                 var nbr_buckets = context.worst_cost + context.worst_heuristic_cost + 1
 102                 var buckets = new Array[List[N]].with_capacity(nbr_buckets)
 103
 104                 for i in [0 .. nbr_buckets[ do
 105                         buckets.add(new List[N])
 106                 end
 107
 108                 graph.pathfinding_current_evocation += 1
 109
 110                 # open source node
 111                 buckets[0].add(self)
 112                 open = true
 113                 self.last_pathfinding_evocation = graph.pathfinding_current_evocation
 114                 self.best_cost_up_to = 0
 115
 116                 loop
 117                         var frontier_node: nullable N = null
 118
 119                         var bucket_searched: Int = 0
 120
 121                         # find next valid node in frontier/buckets
 122                         loop
 123                                 var current_bucket = buckets[cost % nbr_buckets]
 124
 125                                 if current_bucket.is_empty then # move to next bucket
 126                                         cost += 1
 127                                         if cost > max_cost then return null
 128                                         bucket_searched += 1
 129
 130                                         if bucket_searched > nbr_buckets then break
 131                                 else # found a node
 132                                         frontier_node = current_bucket.pop
 133
 134                                         if frontier_node.open then break
 135                                 end
 136                         end
 137
 138                         # no path possible
 139                         if frontier_node == null then
 140                                 return null
 141
 142                         # at destination
 143                         else if frontier_node == destination or
 144                              (alt_targets != null and alt_targets.accept(frontier_node)) then
 145
 146                                 var path = new Path[N](cost)
 147
 148                                 while frontier_node != self do
 149                                         path.nodes.unshift(frontier_node)
 150                                         frontier_node = frontier_node.best_source.as(not null)
 151                                 end
 152
 153                                 return path
 154
 155                         # adds all next nodes to frontier/buckets
 156                         else
 157                                 frontier_node.open = false
 158
 159                                 for link in frontier_node.links do
 160                                         var peek_node = link.to
 161                                         if not context.is_blocked(link) and
 162                                          (peek_node.last_pathfinding_evocation != graph.pathfinding_current_evocation or
 163                                            (peek_node.open and
 164                                              peek_node.best_cost_up_to > cost + context.cost(link)))
 165                                         then
 166                                                 peek_node.open = true
 167                                                 peek_node.last_pathfinding_evocation = graph.pathfinding_current_evocation
 168                                                 peek_node.best_cost_up_to = cost + context.cost(link)
 169                                                 peek_node.best_source = frontier_node
 170
 171                                                 var est_cost
 172                                                 if destination != null then
 173                                                         est_cost = peek_node.best_cost_up_to + context.heuristic_cost(peek_node, destination)
 174                                                 else if alt_targets != null then
 175                                                         est_cost = peek_node.best_cost_up_to + alt_targets.heuristic_cost(peek_node, link)
 176                                                 else est_cost = 0
 177
 178                                                 var at_bucket = buckets[est_cost % nbr_buckets]
 179                                                 at_bucket.add(peek_node)
 180                                         end
 181                                 end
 182                         end
 183                 end
 184         end
 185 end
 186
 187 # Link between two nodes and associated to a graph
 188 class Link
 189         type N: Node
 190         type L: Link
 191
 192         var graph: Graph[N, L]
 193
 194         var from: N
 195         var to: N
 196
 197         init(graph: Graph[N, L], from, to: N)
 198         do
 199                 self.graph = graph
 200                 self.from = from
 201                 self.to = to
 202
 203                 graph.add_link(self)
 204         end
 205 end
 206
 207 # General graph
 208 class Graph[N: Node, L: Link]
 209         var nodes: Set[N] = new HashSet[N]
 210         var links: Set[L] = new HashSet[L]
 211
 212         fun add_node(node: N): N
 213         do
 214                 nodes.add(node)
 215
 216                 return node
 217         end
 218
 219         fun add_link(link: L): L
 220         do
 221                 links.add(link)
 222
 223                 link.from.links.add(link)
 224
 225                 return link
 226         end
 227
 228         # used to check if nodes have been searched in one pathfinding
 229         var pathfinding_current_evocation: Int = 0
 230 end
 231
 232 # Result from pathfinding, a walkable path
 233 class Path[N]
 234
 235         var total_cost: Int
 236
 237         var nodes = new List[N]
 238
 239         init (cost: Int) do total_cost = cost
 240
 241         var at: Int = 0
 242         fun step: N
 243         do
 244                 assert nodes.length >= at else print "a_star::Path::step failed, is at_end_of_path"
 245
 246                 var s = nodes[at]
 247                 at += 1
 248
 249                 return s
 250         end
 251
 252         fun peek_step: N do return nodes[at]
 253
 254         fun at_end_of_path: Bool do return at >= nodes.length
 255 end
 256
 257 # Context related to an evocation of pathfinding
 258 class PathContext
 259         type N: Node
 260         type L: Link
 261
 262         var graph: Graph[N, L]
 263
 264         # Worst cost of all the link's costs
 265         fun worst_cost: Int is abstract
 266
 267         # Get cost of a link
 268         fun cost(link: L): Int is abstract
 269
 270         # Is that link blocked?
 271         fun is_blocked(link: L): Bool is abstract
 272
 273         # Heuristic
 274         fun heuristic_cost(a, b: N): Int is abstract
 275
 276         fun worst_heuristic_cost: Int is abstract
 277 end
 278
 279 #
 280 ### Additionnal classes, may be useful
 281 #
 282
 283 # Simple context with constant cost on each links
 284 # Warning: A* is not optimize for such a case
 285 class ConstantPathContext
 286         super PathContext
 287
 288         redef fun worst_cost do return 1
 289         redef fun cost(l) do return 1
 290         redef fun is_blocked(l) do return false
 291         redef fun heuristic_cost(a, b) do return 0
 292         redef fun worst_heuristic_cost do return 0
 293 end
 294
 295 class WeightedPathContext
 296         super PathContext
 297
 298         redef type L: WeightedLink
 299
 300         init(graph: Graph[N, L])
 301         do
 302                 super
 303
 304                 var worst_cost = 0
 305                 for l in graph.links do
 306                         var cost = l.weight
 307                         if cost >= worst_cost then worst_cost = cost + 1
 308                 end
 309                 self.worst_cost = worst_cost
 310         end
 311
 312         redef var worst_cost: Int
 313
 314         redef fun cost(l) do
 315                 return l.weight
 316         end
 317         redef fun is_blocked(l) do return false
 318         redef fun heuristic_cost(a, b) do return 0
 319         redef fun worst_heuristic_cost do return 0
 320 end
 321
 322 class WeightedLink
 323         super Link
 324
 325         var weight: Int
 326
 327         init(graph: Graph[N, L], from, to: N, weight: Int)
 328         do
 329                 super
 330
 331                 self.weight = weight
 332         end
 333 end
 334
 335 # Advanced path conditions with customizable accept states
 336 class TargetCondition[N: Node]
 337         # Should the pathfinding accept `node` as a goal?
 338         fun accept(node: N): Bool is abstract
 339
 340         # Approximate cost from `node` to an accept state
 341         fun heuristic_cost(node: N, link: Link): Int is abstract
 342 end